1. Field of the Invention
The present invention relates to an exposure control device for a camera, particularly with an electric control circuit for automatic exposure control, and more specifically to an exposure control device capable of programmed automatic exposure for automatically controlling both exposure time and diaphragm aperture according to a predetermined program.
2. Description of the Prior Art
There are known programmed exposure control devices capable of detecting an exposure value (EV), prior to the photographing operation, from the information on luminance of the object obtained from a light metering device, determining a combination of the time value (TV) and the aperture value (AV) from thus detected exposure value and an ASA speed value (SV) according to a predermined program, and automatically controlling a shutter mechanism and a diaphragm mechanism from thus determined combination. A programmed exposure control device disclosed in the U.S. Pat. No. 4,204,755 performs a programmed exposure control for the luminance value within a determined range in which the time value and the aperture value are changed at a determined rate, but changes the time value alone while fixing the aperture value at a limit value for a very large or very small luminance value outside the limits of control range for the aperture value.
For photographing purposes with appropriate exposures, such conventional device allows expansion of the photographable range of luminance value by changing the time value even when the aperture value reaches a limit.
The above-mentioned reference also discloses a display device for thus controlled exposure, but such display device only displays the shutter speed not the diaphragm aperture. On the other hand a conventional camera capable of displaying both the shutter speed and the diaphragm aperture requires independent display devices for such displays. A display device composed of light-emitting diodes consumes significant electric power for display, thus reducing the service life of the battery. On the other hand, a display device composed of liquid crystal elements is not associated with such drawback in the power consumption. Such liquid crystal display should preferably be used in the static drive mode in consideration of the display on the camera in a low temperature condition, but such mode for two displays for the diaphragm aperture and the shutter speed requires increased wiring which cannot be easily accommodated in small equipment such as a camera. In addition the use of two display devices leads to increased cost for the driving circuits as well as for the display devices themselves.